On the intake side of the rotor blades, during operation, a boundary layer is formed. In the case of a blower, the boundary layer is subjected to a positive pressure gradient created by the blower, and it follows from this, that the boundary layer can detach. The disadvantages of such a detachment of the boundary layer are higher resistance, and stall. The efficiency of the blower is thereby negatively influenced, and its acoustic noise emissions increase.
In order to hinder the flow detachment or to displace the beginning of the flow detachment in the direction of the trailing edge of the respective blade, the so-called “turbulators” were developed. This term means measures for inducing a changeover from laminar flow to turbulent flow. The advantage of a turbulent rather than a laminar boundary layer lies in its higher kinetic energy, which enables a greater pressure increment without detachment. However, such turbulators make no sense for fast-turning blowers since, in their case, the flow is turbulent anyway.
It is therefore an object of the invention to provide a new blower with improved characteristics.
This object is achieved by forming a trip edge, running from a radially inner side of a rotor blade to a radially outer side thereof, having a generally S-shaped contour and located, measured from the leading edge of the rotor blade, within a band spanning from about 30% to 100% of the length L from leading edge to trailing edge of the blade.
For improvement of the flow relationships on the blade surface, the invention employs a discontinuity of the blade surface. In technical terminology, this is called, for example, a breakdown edge, a step or a “trip-wire.” Conventional trip-wires extend radially and cause—in the case of slow-turning blowers—adjacent the leading edge of the moving blade, a changeover from laminar to turbulent flow. It has been shown that, with such a trip-wire, the flow will detach as function of the load on the moving blade on the intake or suction side of the blade profile. From this point, the profile is surrounded by a suddenly thicker-becoming, non-contacting and uncoordinated flow. For the neighboring flow, this has the same effect,
as if the moving blade were significantly thicker. Thereby, the blade channels of the blower become partially or completely blocked, and the delivered volumetric flow therefore diminishes. By “blade channels,” one understands the passage between two adjacent fan blades.
For this reason, the form of the contour of an optimized trip edge follows the contour of the detachment zone in the vicinity of the trailing edge of the respective blade. Thereby, the boundary layer at the point, at which the detachment would begin, is supplied with additional energy. Behind the step, a recirculation zone, consisting of micro-eddies, forms, over which the adjacent flow can glide with minimal friction. In contrast to conventional trip edges, the detachment zone either locates itself more strongly toward the trailing edge of the relevant blade, or the detachment zone is completely eliminated. The following advantages thereby result:                Pressure increase of about 6.3%        (calculated via CFD-simulation)        Reduction of noise energy in the free-exhausting range, thus reduction of fan noises.(CFD-Simulation Means Simulation by Computational Fluid Dynamics)        
By a contour-optimized trip edge on the suction side of the relevant fan blade, an expanded or stretched recirculation zone forms downstream, along the blade surface (on the suction side). This reduces friction for the fluid layers passing thereover. Thus, the boundary layer can be provided with renewed kinetic energy. Its energy equilibrium settles in a stable range, and the flow detachment is displaced into a downstream-lying zone. This lengthens the effective region of the rotor blade and guides the flow, corresponding to its exit contour, over this rotor blade. Since the rotor blade has nearly optimal flow adjacent it for the entire operating range of the blower, the noise energy, emitted by the blower in the regions away from the designed operating point, is reduced.
The form of the trip edge is, in the ideal case, matched to the contour of the detachment zone at the design point, and it describes a curve parallel to the contour of the detachment area with a spacing DS approximating 1 to 2% of the diameter D of the fan wheel. This is necessary, in order to achieve effectiveness in the operating states which deviate from the design point. In general, the trip edge thus has the form of a stretched S, which runs approximately parallel to the trailing edge of the respective rotor blade.
Further details and advantageous refinements of the invention will be apparent from the embodiments described below, and shown in the drawings, which in no way are to be understood as limiting the invention.